[ibis-macro] another silly IBIS-AMI question about deriving the impulse response of a channel

From: Scott McMorrow <scott@xxxxxxxxxxxxx>

To: IBIS-ATM <ibis-macro@xxxxxxxxxxxxx>

Date: Thu, 27 May 2010 18:03:43 -0400

Since the analog channel is required to be LTI from driver to receiver,
I believe that in order to create a correct impulse response it is
necessary to know what the termination impedance should be. In the case
where a driver or a receiver is terminated at the end of the line, the
termination impedance is obvious. So, for a 100 ohm differential system
the differential impedance used for impulse response extraction is 100
ohms. Essentially the high-impedance receiver amplifier, or driver
current sources are considered to be lumped at the termination network.

How are people modeling, extracting, and simulating drivers and
receivers where the high-impedance ports are physically isolated from
the termination, where the waveform in the algorithmic domain
(driver/receiver) is substantially different than the waveform in the
analog domain at the termination? I'm confused, since IBIS-AMI does not
include the on-die termination network, except in the rudimentary and
unsatisfying form of a *.ibs model, and that model is a lumped
approximation. In my mind, in order to correctly extract an impulse
response of a channel that will give a correct representation of the
waveform at the receiver, one has to either perform a transient
simulation of the full circuit from the driver to the receiver, or
perform frequency domain modeling to impulse response transformations
with known driver/receiver input impedance. Otherwise, the resulting
impulse response is wrong.

For a hypothetical 50 ohm single-ended transmitter or receiver, the
simplest circuit that can correctly model many transmitters and
receivers is the following network: